Positive-displacement reciprocating pumps designed for cryogenic liquids, or cryogenic reciprocating pumps, are commonly used on portable equipment for oil field service and industrial applications where nitrogen is delivered as a cryogenic liquid, pumped to a higher pressure, vaporized, and then injected into a well, pipeline, vessel, or otherwise delivered for end use. Many of the commercially manufactured designs are comprised of a warm end, multiple cold ends in parallel, and intermediate distance pieces connecting each cold end to the warm end.
The warm end is further comprised of a housing, crankshaft, connecting rods, and crossheads that translate rotary motion to linear motion for the cold ends. The cold end is the pump body that comprises a plunger or piston, a cylinder, cylinder head, suction valve, and discharge valve. The intermediate distance pieces thermally isolate the warm end from the cold ends while aligning the cold end piston with the warm end crossheads.
The common commercial cold end designs have piston packing seals that are located just beyond the piston stroke length from the cold end cylinder. The piston packing seals prevent low pressure cryogenic nitrogen from leaking to the atmosphere and potentially spraying cryogenic nitrogen on the warm end crosshead oil seals that cannot tolerate cryogenic temperature.
The cold end piston operates below the freezing point of water, thus, ice can form on the portion of the cold end piston that is exposed to ambient air within the intermediate distance piece. A metallic scraper, also referred to as a wiper, may be positioned adjacent to the piston packing seals opposite from the cold end cylinder to clean contaminants, primarily ice, from the piston. The wiper is meant to protect the piston packing seals from physical damage from ice accumulation and other contaminants. The wiper has been proven to be effective when the cryogenic reciprocating pump is operated at a speed in the upper portion of its design envelope, but the wiper has proven to be ineffective when the cryogenic reciprocating pump is operated at lower speeds.
The piston packing seals are often plastic materials, commonly blends of Polytetrafluoroethylene (PTFE) and structural modifiers such as fiberglass or carbon. These materials are suitable for service at cryogenic temperatures, but have a thermal contraction rate much greater than the cold end piston that the piston packing seals surround. The difference in thermal contraction increases the stress in the piston packing seals at low temperatures resulting in increased cold flow deformation
The design of many commercial cryogenic reciprocating pumps is a suitable compromise for many applications, particularly when the pump is operated for periods substantially less than ten hours before allowed to derime, or when the pump is rotated in the upper half of its design speed range. Their design, however, results in common issues when the pump is operated at lower speeds for an extended period of time. In continuous operation, ice formation on the cold ends and intermediate distance pieces continues to build up over a period of time. The ice buildup insulates the portions of the cold end and the intermediate distance piece surrounding the piston packing seals, and the temperature of the piston packing seals continues to decrease over hours after beginning continuous operation. An extended duration at cold temperatures contributes to deformation in the piston packing seals that prevents them from sealing when warmed up again. Furthermore, the common wipers have proven to be an effective measure to clean all condensation and frost resulting from exposure to ambient water vapor from the piston when operated at sufficient speed, but wipers, even in good condition, are often unable to remove hard rime that forms on the piston at low pump speed.
Previous cold end designs have included means for keeping the piston packing seals substantially warmer than the pumped fluid. Such features of various designs include elongated dimensions to reduce heat conducted from the piston packing, fins surrounding the piston packing to increase the transfer of heat from ambient air to the piston packing, insulating sections to thermally isolate the piston packing from the cold temperature within the pumping chamber, and a piston packing seal warming fluid jacket integral to the housing surrounding the piston packing seals. The drawback to these features is that they generally increase the dimensions of the cryogenic reciprocating pump, which is undesirable for mounting on a truck or trailer, and they make replacement of the cold ends more cumbersome.
Some features of the traditional cryogenic reciprocating pump designs emphasize reducing heat transferred into the cryogenic fluid as it is pumped in order to reduce vapor that must return to a cryogenic storage tank. Vapor returned to the tank increases the temperature of the stored cryogenic fluid, reducing the net positive suction head available to the cryogenic reciprocating pump. The returning vapor may also be vented directly to the atmosphere due to the operating pressure of the cryogenic storage tank. These features restrict heat transferred from the warm end into the cold end, and sometimes reduce heat transfer directly from ambient air through the cold end housing into the pumping chamber with a vacuum-insulated section.
Many of the commercial cryogenic reciprocating pumps designated by manufacturers for oil field service applications (e.g., ACD, NOV HydraRig, CS&P Technologies) do not use similar design features to limit heat transfer into the cold end because the equipment incorporating the cryogenic reciprocating pump typically also incorporates a cryogenic centrifugal pump to increase net positive suction head available to the cryogenic reciprocating pump. Furthermore, when vapor generated within the cryogenic reciprocating pump is vented to atmosphere, the amount is insignificant in comparison to the relatively high design rates of many cryogenic reciprocating pumps marketed for oil field applications.
The cold ends of cryogenic reciprocating pumps marketed for oil field service applications commonly allow liquid nitrogen within the cold end housing to be in direct contact with the piston packing seals. These pumps are designed to prevent excessive heat transfer from the warm end through the intermediate distance piece into the cold end to prevent freezing of lubricating oil within the warm end, but these designs do not incorporate any mechanism or feature to keep the piston packing seals well above the temperature of the cryogenic fluid. These designs of the cold ends marketed for oil field service also do not allow extended heat transfer surface area or a heating jacket on the cold end for the piston packing seals because the piston packing seals are installed in the section of the cold end housing that is immediately surrounded by the intermediate distance piece. Thus, the piston packing seals of cryogenic reciprocating pumps for oil field applications undergo repeated thermal expansion and contraction while restricted by adjacent parts within the cold ends, and the piston packing seals deform. Deformation in the piston packing seals compromises the ability to seal the fluid within the cold end housing.
Thus, there is a need in the art for a means to warm the piston packing seals in cryogenic reciprocating pump cold ends in which the piston packing seals are in close proximity with the cryogenic fluid, and in which there is no means to improve the cold end to warm the piston packing seals. The means to warm the piston packing seals is needed to increase the life of the piston packing seals when operated continuously and at low operating speeds.